1. Field of the Invention
The present invention relates to an image reader having a plurality of photosensors and, more particularly, to an image reader having a means for correcting variations in photoelectric conversion outputs from photosensors.
An image reader according to the present invention is widely applied to an image input apparatus such as a facsimile system, a digital copying machine, or the like, and input units for other image processing apparatuses.
2. Related Background Art
In an image reader, an original as an object to be read is irradiated with light, so that reflection light or transmission light becomes incident on a plurality of photosensors so as to be ready by them. Thus, image information of the original is output as an electrical signal.
In this case, photoelectric conversion characteristics of these photosensors slightly vary from each other. As emphasized on the graph in FIG. 4(A), photoelectric conversion outputs from the photosensors vary even with an identical original concentration.
In a conventional image reader, in order to correct the output variations and to obtain an electrical signal accurately corresponding to image information of an original, a reference white original is read, and a photoelectric conversion output is stored as correction reference data. A photoelectric conversion output of a normal original is corrected using the correction reference data, thereby obtaining a uniform output.
However, in the conventional image reading, an original which is approximate to reference white does not cause variations in outputs, and an image of the original can be accurately converted to an electrical signal. However, when an original has a high concentration and a low reflectivity or when an output is attenuated due to a change in temperature or aging, variations in output levels after correction become large, and an electrical signal which precisely corresponds to the image of the original cannot be obtained.
As shown in the graph in FIG. 4(B), since the photoelectric conversion outputs are corrected with reference to the reference white, they can be accurately corrected according to the correction reference data, and output levels among the photosensors are rendered uniform. However, as the concentration of an original becomes high and its reflectivity is decreased or as the outputs are decreased due to a change in temperature or aging, the outputs cannot be accurately corrected by the correction reference data, and variations in output levels among the photosensors become large. For this reason, image reading is inaccurate, and this results in poor reliability.